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Abstract

We demonstrate that silicate bonding an optical flat to the output facet of an active fiber device can increase the reliability of high-peak power systems and subsantially reduce the effective feedback at the termination of a double-clad fiber. We determine the bonding parameters and conditions that maximize the optical damage threshold of the bond and minimize the Fresnel reflection from the bond. At 1-μm wavelength, damage thresholds greater than 70 J/cm2 are demonstrated for 25-ns pulses. We also measured Fresnel reflections less than -63 dB off the bond. Finally, we determined that the strength of the bond is sufficient for most operating environments.

Figures (13)

(a) Illustration of an optical flat bonded to the end of a fiber. The fiber core and the fiber cladding are made of fused silica. The jacket or buffer layer is made of either a polymer or fused silica. (b) Photograph of a double-clad fiber that is silicate bonded to a 1”-diameter optical flat. The fiber has an inner cladding of 250 μm and a low-index acrylate jacket with a diameter of 450 μm. The exterior of the last few centimeters of the fiber’s length is epoxied to the inner wall of a thick-walled capillary with a 475-μm inner diameter and a 6-mm outer diameter. The 6-mm diameter capillary is used to provide mechanical support to facilitate polishing and subsequent bonding.

Coreless end cap spliced to the gain fiber. The end cap is polished at an angle to suppress feedback. The polish angle and the angle of the free-space output beam with respect to the fiber axis are exaggerated for clarity.

Experimental setup for measuring microsecond damage threshold of silicate bond.
The laser system produces 100-mJ, 880-ns pulses at 1064 nm. AOM: acousto-optic modulator, HWP: half-wave plate, QWP: quarter-wave plate, PBS: polarizing beamsplitter, AWG: arbitrary waveform generator, HR: high reflector. Included is a photograph of the damage induced by the microsecond pulse in one of the samples. The diameter of the outer circle shown is approximately 420 μm.

Experimental setup to measure Fresnel reflections from silicate bonds. The pulse width of the 10-W Nd:YAG mode-locked laser is 3 ps, which is considerably shorter than the 20-ps resolution of our oscilloscope.

Traces of the reflections from various interfaces in silicate bonded samples. Plot (a) shows the traces on a linear scale for a sample with a measurable bond reflection (solid, black) and for a sample with a bond reflection below the detectable limit of the experimental setup (dotted, red). Plot (b) shows the trace for the sample with the undetectable bond reflection on a logarithmic scale.

Measured reflection off of silicate bond versus different (a) solution concentrations (b) solution volumes and (c) curing temperatures. The median reflection value for each group is plotted. The dotted line indicates the sensitivity of the measurement.

(a) Setup for measuring shear strength of silicate bonded samples. Weights from 5 kg to 25 kg were used to apply stress in our setup. (b) Photograph showing one of the silicate-bonded samples that fractured.